A myriad of non-contact imaging modalities may be utilized in generating three-dimensional images of opaque objects. Systems such as laser scanners, interferometers, projected pattern analyzers, and the like, are well known in the imaging art.
More recently, Holographic Laser Radar (HLR) has been utilized to produce three-dimensional images as described in detail in U.S. patent application Ser. No. 08/282,781, entitled "System and Method for Three-Dimensional Imaging", filed on Feb. 4, 1994, assigned to the assignee of the present invention. HLR is an interferometric technique that illuminates an object with laser radiation of different frequencies and measures the far-field speckle pattern for each of the illuminating frequencies. Appropriate selection of the laser beam bandwidth provides the desired range resolution for the system. The resulting laser speckle pattern is characterized by in-phase and quadrature measurements which form a three-dimensional Fourier-volume (or aperture) representation of the illuminated object.
The three-dimensional imaging systems described thusfar each have associated disadvantages. For example, laser scanning systems require a cross-range scanner and a precision focusing element which can be expensive. Furthermore, the sequential scanning process requires considerable time which poses a problem in imaging objects having a large cross-range space-bandwidth product, or objects which evolve over the scanning time. Furthermore, the cross-range resolution of the three-dimensional image is limited by the quality and size of the focusing element, which presents a problem particularly in long-range imaging.
Interferometric systems are often used to measure surface profiles of objects but are typically used for specialized applications, such as optical testing. Range ambiguity is often problematic for such interferometric systems. Projection of regular patterns on three-dimensional objects to analyze object deformation or the like, requires an elaborate pattern projection system which is also difficult to use in long-range imaging applications. The HLR sensor is an interferometric sensor which requires measurement of complex fields for the speckle images at various frequencies in order to record the phase relationships both spatially, and among those frequencies. Furthermore, precise alignment of optical components and precise phase stability of both the illumination source and the interferometric component are also required. These requirements are difficult to establish and maintain in long-range imaging applications or imaging in stressing environments, such as when mechanical vibration is present. In addition, aberrating media near the detector plane will cause the phase of the measured field to be aberrated resulting in a corresponding reduction in quality of the three-dimensional image.